Partner mentions they are having to stay late for work drinks one night in the week.
You then see emails from your partners work email account arranging when and where to meet. It’s clear from the context it’s only your partner and one other person, and the other person doesn’t work for your partners organisation (although obviously does work in the same sector). No one else is going - there was supposed to be a third person but emails show they dropped out, but drink still going ahead. Your partner is heterosexual and the person they are meeting is the opposite gender. You don’t recall them mentioning this person before.
Would this give you pause? Would you wonder why your partner hadn’t explained it was a drink with one other person? Are you happy for your partner to meet non-colleagues for drinks on a one on one basis? Would you expect them to tell you that’s what they were doing first?