Soya juice (what she used) probably has the advantage of not smelling. Onion and lemon would give themselves away, I feel!

It looks like any organic liquid works, including milk in the case of Lenin, soya juice in your case, lemon and onion if you want to tip off the spied ;)

I’m curious about how it bends the paper. The retractile nature of drying juices must create visible irregularities on the surface.