Russian plane shot down in Syria, page-47

All Russian military aircraft signitures be it transponders or not would be linked to their air defense network which is linked to the Syrian air defense system for friendly identification purposes. All militaries do this, it is common practise. The transponder would have been on for the Russian aircraft as they are not at war and the aircraft is a for survalence purposes. It's out there deliberately to be seen by all parties.

We are not talking about some stealth figher here flying around at mach 2 or more. The aircraft we are talking about goes the speed of a lumbering airliner.