My FDC lady will not allow any visitors to the house when she's got kids. The exceptions to this is if they have relatives visiting from interstate which has happened a few times, but she always introduces them and always tells me at least a few days before they arrive that they are coming.
Our carer and her husband are in their late 60's or early 70's at least and the lady is the qualified carer. While the husband is home, he doesn't look after the kids, or even hold them, with the exception of their grandson who is 2 and is one of the kids they look after.
Occasionally their youngest daughter (who is probably in her late 20's early 30's) lives at the house but she is rarely home when the kids are there as she works. Everyone who lives in the house has been police checked and any time they have visitors staying from interstate they always notify the head office.

In your case, I think you have every right to know who the person was and how often they visit. I'm just not sure how you would word it to ask.