What is the oily liquid under my MacBook Pro's heatsink?

When I took my MacBook Pro 13-Inch "Core i5" 2.9 Early 2015 to Apple to have the battery and screen replaced they opened it and found that the LCI next to the heatsink had been triggered. They also discovered an oily liquid between the rubber cup (located below the heatsink when the MacBook is orientated for use) and the conducting arm that transfers heat from the processor to the heatsink itself. (Note the rubber cup is removed in the attached photos).

Present in only two small patches, this oily liquid simply could not have entered the MacBook externally or it would have made a mess all over the inside of the MacBook… So the source must be internal.

My question is therefore: What could this liquid be?

The MacBook is used mainly in an office environment but has travelled a lot. It has however seen significant but not excessive use for most of the four years I have owned it. And in the past three months, whilst I have not been able to replace the battery (for practical / business reasons) It has been used on mains power only because the battery is not operational - hence the need for repair.

Any thoughts or suggestions welcome.

I would also welcome advice on how to convince Apple to accept the MacBook under its discounted battery replacement programme and MacBook Pro Anti-Reflective Coating Repair Program, for which my MacBook would otherwise be eligible - at least for a few more months.

Thanks