So why do you keep saying you do know?
Nothing about this has suggested they are doing anything wrong. If someone is out and about at the weekend that is not doing anything wrong. Pushing to investigate someone just going about their business is not ok.
Staff conduct is important but what conduct is being broken by being out in public? Absolutely nothing!
Would you want all staff investigated because they met up with a colleague? There are many meetings with staff - weekly events in most schools. Staff go to each other's houses, sleep over, go on holidays together, go to events like the races, go out drinking, go shopping, go to each other's weddings, hen/stag dos, babysit for each other, help decorate, help run businesses, plan together, games nights, do joint training, be each others support in times of need.
You cannot police the meet ups because it is a male and a female meeting and you think that males and females are up to no good if they meet up due to your own sad little judgements on male/female friendships.
What if two female staff met up? Would that be a cause for concern? If not, why not?