A couple of days this week I've noticed a bit of water in the drip tray in the morning, and the pump has run a bit longer to fill when initially turned on.

This morning I realized that it seems to have dripped from the group overnight. When the machine is on and warm there is no dripping.

I'm assuming this would have something to do with the piston seals? Any thoughts (other than to replace)?

The piston seals are most likely culprit.