Source of grease inside unibody macbook?

I have a unibody 2012 macbook which appears to have some kind of grease around the battery area, between the battery and the case.

This machine has been serviced by a third party applecare provider for hard drive cable failure several times, and I'm wondering if that's something they might have left in there for some reason?

Are there any pads/components/labels that sometimes leach grease under high temperature?

The only other plausible explanation would be that it has somehow leached in through the screw holes (there is no contamination in the keyboard/motherboard/optical area, so it can't have come from above), which I also find pretty hard to believe.

Visually it looks like a sticky drink has dried up, but when you touch it it has the consistency of vaseline.

Picture (apologies for poor quality, phone is all that's available):

All plausible theories appreciated!