I was doing some pre-op on my z500 last evening preparing to ride today and it dawned on me that i hadn't checked my coolant level since a couple of rides ago when i replaced the fan thermo switch.  When I removed the hood I found the overflow tank cap was off and the plug insert laying on top of the tank as if i'd simply left the cap off or something like that, but I'm pretty sure I didn't.  The tank was almost empty, but the radiator was full, and there was no leaks anywhere that I could find. The last time we rode the temperature guage seemed to malfunction and stayed on one bar most of the time which I think repaired (dirty pin connectors on the sensor). I filled the overflow, put the insert and cap back on. Any ideas? I'll be riding today so we shall see...

[2015 Z Force 800 53/Trail EPS]

The tank is vented so there's no way I know of for it to get enough pressure to come off...so..gremlins?  My only concern now is that not only does the bottle catch any that bypasses the pressure cap.. but as it cools-down and makes vacuum it pulls fluid back in to maintain a completely full system. With the top off while riding it is likely dust and sand got into the bottle and although it will probably settle out when not in motion, it may have enough in solution to be pulled into the system at some point. And when that gets caught between the two parts of the water pump's mechanical seal,...well, let's just say it's life expediency will be quickly altered.

I might be tempted to pull and clean the bottle...that's just me...and make sure there is no air gap between the coolant and cap in the radiator.   

Yeah gremlins are at fault! I must have left the cap off the tank, but there was no debris in the the tank so with the insert setting on top maybe it stayed in place while riding? Anyhow we rode 60 miles today, mostly in low range, up hill, and on very rocky terrain. We had no problems whatsoever. I'll keep a check on it though, thank for the input NMK.