Good one. In my case it was a microscopic pinhole in the radiator hose. It never overheated, never a leak, but the fill tank would empty every 6 months. This went a couple of years, then I happened to pop the hood while running to check a squeak or something right after a run and noticed the teeny stream for only 10 seconds and vaporizing right onto a little green "mossy" looking spot on the block.
\'96SS SOB: SSRI, Herter Tune, Tri-Y II, SLP SS, 3000 Edge, F/HO bars, METCO extendeds and CV MMC, Bilsteins, currently Vredestein Ultrac Sessanta (315/35 rear)
\'96FWB BUW-Butt Ugly White: SS Bars, METCO LCAs, Koni Reds, PowerTrax No-Slip