This discussion is sort of about a different thing, but to answer your questions:
Is there any indication of a probe launch? Graphical, audible?
Nope, it's just an auto-cast ability of scout ships.
I've selected this tech tree update several times and I am not sure I've seen it working. If I have my scout on auto roam will it not deploy these probes?
You won't see it deploying, but you'll know you have a probe on a planet if: 1) Mousing over the planet, the info card will say you have a probe at that planet if you do and 2) You'll be able to see ships passing through without having any of your ships in its grav well.
If I have the probe ability on auto does the auto roam ability override the auto probe launch?
Nope, you can have both enabled, one doesn't override the other. But deploying probes has a rather long cooldown and a rather high antimatter cost, so if you leave your scout in auto-explore mode it won't be able to drop a probe at each planet it passes on the way.