If you must add a 'stop leak' type product then use either a "Bars" or "Wynn's" products - these both work in a similar way to each other (but totally different to how Radweld works) by only solidifying were they find an actual leak. In fact Wynn's (an American company) do some very good additives etc.
Certainly a pin hole leak in the rad, or even hose, does make sense - as TonyB says, whilst the cooling system is under pressure it will leak, as the water cools it will draw in air but being such a small hole it might not be very obvious.