I agree, likely switches if it's wood ties. Check for debris near the point rails of course. A single loose wire strand broken off and landing in the wrong spot sounds very plausible as a foriegn cause. Try working the switches to move debris and then leaving them in the opposing position. One by one so you know which needs attention. Don't group fix attempts or you won't know what you did right. Process of elimination takes diligence.
If the track is ballasted, or attached in way that would be a pita to remove, consider cutting the center rails via dremel to create a block for each switch. Reconnecting the blocks by wire afterwards if everything checks out ok.
The "Lemonade": Should anything like this happen again, it will be much easier to check quickly if each switch is on it's own block.
Using terminal blocks vs splices in layout design also helps with speeding diagnostics like this. Few folk like to think about that though. If they aren't there, you may want to add some while doing this.
The Z4 flashing but not tripping tells me it's very light short, the Z4 likely thinks it's just a heavy loco draw (a single wire strand could do this). As mentioned, it may not be readable by ohms. It's no gaurantee that you can find it by voltage drop either, but testing with low voltage and slowly increasing it slightly as you test may help.
I don't know where he got it or who made it, but a pals dad, now passed, had an automotive short detector that could find even partially damaged wire inside insulation. It was as simple as grounding it and passing the tip along a wire. I will call later to ask more, but the son has strange work hours, and might not call back until Sat. (I want to know more anyhow, he passed a few days after I saw it used.) It likely uses induction or a special circuit to perform Leo's phone static trick (which is pretty cool 
, right up there with seeing IR light with a digital camera/phone )
