Whilst you might carry everything around with you, not everyone does. My wife takes only the minimum everytime she leaves the house. Often just her phone! She won't even take her house keys if she knows I'll be in when she gets back.
It's not so much emptying everything out, as just putting the bare minimum in. It would never occur to her to take her driving licence anywhere.
Most people, and especially women, do not treat their handbag as an empty vessel by default, only placing things in it they need for that day or that trip, but as a personal space full of stuff by default, with things removed from it to make space.
Put it this way, the OP's partner is going to have to say, outright, that not only was the pass misused by the OP, but also that it was taken without their consent.
I would absolutely not want to be in a disciplinary hearing defending my actions to leave my staff pass at home only on the days I drove into work, and, by denying collusion with my partner in the misuse, directly accusing my partner potentially of another offence.
"And why did you leave your staff pass at home on those days, the same days your partner misused it?"
"Well you see I don't carry everything I need all the time, I took it out"
follows:
"So your bag is usually completely empty is it, and you only take things you need for that day?"
"Isn't the pass important? Why would you take it out?" "As you know from the employee handbook/conditions of the pass, misuse of the pass is very serious. You've signed to say you read the conditions - so can you confirm you understand misuse of the pass is a serious matter, and that if you are complicit in that misuse, it may be construed as serious/gross misconduct (delete as appropriate)?"
"Thank you. In light of that, why
did you leave it somewhere it could have been misused?"
"Do you trust your partner?"
"Why didn't you apply for a nominee pass?"
"Did your partner steal the pass from you to misuse it?"
"Are you sure the pass was taken without your knowledge? That's got very serious consequences for your partner. We might need to inform the police/revenue protection of this information, for the benefit of the prosecution's case."
"Would you like us to take a break while you think about your answer to that question?"
etc etc etc
Most of the answers to those questions are quite difficult, and things could go south pretty fast in the disciplinary hearing, which, we have already ascertained, will be without union representation (another very bad decision not to join the union, I'm afraid!).
One of the things which is obvious from giving advice in this part of the forum is how many respondents don't appear to have been
In Big Trouble anytime in their life, and are blissfully naive of how quickly things can go bad when you start talking, even when you have good intentions!
It is much more important to consider how circumstances
appear, and how your actions
appear, than the objective truth, when in a position where the company has to prove nothing, only that something happened on the balance of probabilities. A raised eyebrow and "sounds like a load of old toss, reckon on balance that's a lie" is enough for HR to undertake a disciplinary remedy, and "more likely to be a lie than the truth even if we can't tell" is also perfectly acceptable at a tribunal. This is not a criminal hearing on the part of the OP's partner, who is on weak ground.