Not very useful to find once you've crushed up half the existing tiling.
Breaking up the 50 year old concrete pad near my house, I found old 1970s crushed beer cans buried under the concrete (the kind that predate the attached pull-tab). Kind of a neat link through time, imagining them sweating in the same sort of heat that I was.
Of course, it might also explain some of the other 'questionable' issues in the construction that I keep finding...