Brake Reservoir Overflow

This is a new one on me. I'm not overly concerned just now given we're talking a few drops at most, but my brake reservoir seems to have overflowed at some point this past week.

You can see where it's run and spoiled the printed text, and it's pooled on the lip at the front.

Level's still sitting on max line for now but i'll keep an eye. The cap was on securely, although with these plastic 2 piece lids i'm sure they can't hold back much pressure even when done up properly.