I've used Randen's method before (do it outside!).
When I was looking for a very hard to find leak on my system, I ended up trawling the bubble blowing forums for the best bubble mixture. Plain old soapy water was not tenacious enough to find the leak (I could pull a 500 micron vac and gain 1000 microns overnight).
I ended up with a mix of dish washing liquid, water and glycerine. The trick turned out to be lots and lots of dish washing liquid. Like a 50/50 mix of soap/water. I just added a tiny splash of glycerine to that and put it into a squirt bottle.
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