I have 4 whites tree frogs - cute little things

The other day we got them out and at least 2 of them seemed to "quirt" water from the back. Not quite sure what it was.

It didnt look or smell like urine. It was clear like water and had no smell.

It was like when you squirt water out of a toy!

i mentioned it to Georgie - he got one out and the same thing happened.

We got them out again yesterday ( all 4) but only 2 did it again.
So i would like to know if anyone knows what this is. Is it a warning to leave them alone, or some sort or mating thing, or is it urine - if so it just seems unusual as there is no smell.

I have looked in the few books i have but cannot find anything - unless i am looking in the wrong direction.
Any ideas guys????????
Lin