Ask if they want to alter your schedule. Say I've noticed you've been needing me to stay late! I don't mind at all bit I was just wondering if you wanted to alter our working agreement at all to make up for that time. That gives us all a better idea of timing.
My boss was 5 mins late or more EVERY DAY with out fail(DB). Our contract says I get paid for anytime over our 15 min grace period. It goes both ways. ( however I was asked to start coming 15 mins early everyday but my grace period extends to 8:15)
I asked if they want to alter my schedule. Mb said no. We'll vet since then I have been off on time or if the occasional insadent happens I am contacted right away