The initial suspicion was based specifically on her reaction to our arrival - i.e., immediately declaring us to be her rescuers, which struck me as extremely odd. This initial suspicion dovetailed with your written description of her appearance, which was:
PMiller wrote:A slender woman of medium height and long, black hair sat in a chair at the table looking extremely sad. Her clothing looked well-worn, though it did little to detract from the beauty of her appearance.
I thought that what you wrote supported the picture you chose, and was a deliberate clue on your part to give us insight into her true nature. A person who had been imprisoned in a room "a really long time" would surely have greasy, tangled, and stringy hair, a blotchy and blemish-ridden complexion, foul body odor and bad breath, grimy and possibly broken nails if she had been trying to escape, gaunt features from malnutrition, among other signs - all of which would certainly detract from the beauty of her appearance. So from that, I thought the only sign of her long imprisonment was the well-worn clothing, since you took the time to specifically mention it.
If she does indeed have greasy hair and blotchy skin and the rest, then it is my mistake for assuming. Or if the magical imprisonment, or her "extraplanar nature" has preserved her physical beauty but let her clothing wear normally, then that is also my mistake for assuming.
Aside from the heavy initial clue of her reaction - even if I am to exclude the note about her physical appearance, which I may or may not be correct about - there have been several red flags in her behavior, word choice, and actions that make it very clear that she is deliberately misrepresenting herself and her motivations. I colloquially referred to this as "lying", even if that is an oversimplification of what is actually happening, if her words have been technically true. There have also been further clues to her devious nature, ones that I had again been assuming you were placing deliberately. I thought you were laying the groundwork for her eventual betrayal of the party in a really subtle way, placing near-subliminal clues that would give us reason simultaneously slap our palms to our foreheads, and congratulate you on your excellent storytelling.
Regarding the Sense Motive rolls, I do not choose to let the dice dictate what my character thinks and does, instead relying on observation and logical deduction. Since we do not know if we truly failed our Sense Motive rolls, or if she is telling the truth, but there is a body of evidence that supports that she is deliberately misleading us, I choose to interpret this as that my character does firmly believe Idalla is misrepresenting herself and has bad intentions, but is such a good liar that she shows no sign of it. There is no dissonance there, as both things can be simultaneously true. Hellsing knows that she is not personally an expert in reading body language, so Idalla could be passing off these prevarications in the smoothest and least obvious way possible.
Right now, she is feeling like Idalla presents a clear and present danger to the party, and Hellsing's openly threatening and hostile actions are a deliberate choice. I will say that I believe that if Idalla does approach one of the party members, Hellsing shooting her would not be an evil or chaotic act. I would liken this to the real-world situation of police officers with guns trained on an unarmed suspect. The officers would give strong verbal warnings to the suspect, but if the suspect refuses to comply and approaches one of them, they would shoot the suspect. The officers in question might get some flack for not using a taser, or not attempting a physical takedown, but as they can't be sure if the suspect has a hidden weapon or is a master of krav maga, the use of deadly force would be warranted in most cases in order to keep their fellow officers safe.
In a D&D world, these concerns are even more justified, as a seemingly normal-looking and unarmed person could have invisible magical weapons, spell-like abilities, be intending to cast a spell or summon a weapon, or have some other magical hidden nature that could be deadly.
However, if you are telling me that I am wrong and have been interpreting these observations incorrectly, and are actually harming the game with my character's stance (if, for example, the next part of the game depends on Idalla becoming friendly to the party, possibly giving us information, an item, or aid, or even joining the party as an NPC), then I will have Hellsing walk away and let the rest of the party deal with Idalla themselves. I think you know that I do not like to cause problems in games, and actively work to be agreeable and support others, even if I don't necessarily agree with their direction.