Don't worry about the pressure of pushing a piston into the caliper stressing the brake system. Normal application of the brakes puts a lot more pressure on the system.
Brake fluid has a tremendous ability and willingness to flow (I don't remember the technical term for this characteristic). That's why it's also a good penetrating fluid for stuck nuts and bolts. It is very possible that a concentrated stream from the vent hole shot into the hood pad and spread over time. I'll bet it's still spreading now.
If they had the cap off without a rag, fluid would have spilled over the top of the reservoir and dripped down the master cylinder. If they jammed the piston into the caliper, there might have been enough force into the reservoir to throw a blob of fluid onto the pad. But definitely more around the master cylinder than in the pad.