The key word here is "insignificant". It takes very little for a child to get a bruise - by the way the most common way for a child to get a bruise on their bottom is to fall on a stacking block that has corners - think megablocks.

If the bruise is already turning shades of yellowish or at least lighter in the centre than the edges it is old. If it is still reddish or purple it is new which might help you tell when it happened.

It can also be the parent knowing that it happened at home on the weekend and not sure how to tell you the bruise is there without being afraid of you accusing them of abuse. It works both ways. Try to find out from the parent when they first noticed the bruise - ie if it wasnt' Friday night and they weren't concerned enough about it to contact you immediately then again it likely happened over the weekend from an "insignificant" fall onto a toy.

If the parent was asking the child about the bruise and what happened at your house they may have actually scared the child themselves. Also because of the conversation it made the drop off longer than usual so out of routine, it's monday, lots of reasons for the behaviour but yes conclusions would be drawn for sure.

If the child fell during playtime in the morning then by the time you changed the diaper after naptime there should have been a bruise. A bruise happens immediately because all it is is a small burst blood vessel close to the surface. Over time it gets a bit bigger if the fall is major.

A parent that was truly concerned would either not have brought their child back or certainly not left them after they protested. Assuming the child is happy when they are picked up tonight then over time the parent will relax and so can you.

One of your best defences in a situation like this is to offer quick comebacks like you just know not like you are thinking up excuses. I know the block one because I have had to tell a parent I am pretty sure that is how they got it many times since blocks is one of the first toys beyond the infant toys the toddlers use while they are still in the clumsy stage.