Actually Nigel's point matches yours. When the 'ancients' left things in ponds, shafts etc they left something that was valuable such as spearheads, shields or brooches and suchlike. This would be the equivalent of leaving a 4x4 BMW today!
Leaving cheap and shitty bits of plastic is not showing respect on any level at all. Leaving something of value, such as a tenner would mean far more of an offering.