Got a seemingly-intentional fire in a furniture store.

CCTV-footage shows two suspiciously-acting characters ducking (unfortunately out of camera view)
to two different areas inside an upstairs furniture storeroom;
one amongst stacked mattresses, the other amongst an eclectic collection of sporting equipment.
They quickly duck, and within an hour, an extensive fire breaks out upstairs.
I dug out both areas, discovering nothing definitive, although I did note a sticky green residue on the floor amongst the sporting equipment (didn't smell of anything particular, so I didn't sample it).

A few days later, another fire breaks out at the same furniture store;
firefighters discover a green gummy residue.

Any ideas as to an appropriate time-delay incendiary device leaving such a trace?