I think our real worry from terrorists are the 100 or so ADM (Atomic Demolition Munitions) produced by the now extinct Soviet Union. It is uncertain if all of these ADMs are accounted for... and some may be in circulation for sale to the highest bidder.
These are only 1 kiloton nukes... but still can do a heck of a lot of damage.
Not a chance. Without sophisticated maintenance, the electronics have long since fried, the atomic trigger has decayed into a useless substance, and the conventional explosives in such devices have long since been altered. Likewise, impurities have been introduced into both the core as well as the shell/globe, among other things.
Such unattended (by professionals in top-notch labs) devices will fizzle. The CCCP has been gone since 1989, after all, so even some of their more "recent" devices would have expired long ago.
A terrorist could get a dirty bomb from such things, and a terrorist state or well-financed terror organization could possibly disassemble one of these devices to learn a few secrets, but to try to use such an old, unmaintained device itself is the stuff of Hollywood movies rather than of real science.