Mini-spies are very small Transformers, used by either faction as, well, spies. They are especially suited to this because their faction symbols are not visible without vigorous rubbing.
The Transformers cartoon continuity
A Mini-Spy once blithely drove into Autobot Headquarters, and began messing around with Teletraan-1. Huffer and Cliffjumper rubbed the little robot's rub sign, revealing that he was a Decepticon. The spy fled, with the two Autobots in pursuit. 
Dreamwave comics continuity
- Mini-Spy (1985)
- Mini-Spy toys came packaged with Mini Vehicles in early 1985. Mini-Spies were the first toys to feature rubsigns, and the only way to determine whether a given Mini-spy toy was an Autobot or Decepticon was by rubbing the rubsign.
- There were four different Mini-spy molds, known by the unofficial names 4WD Type, Buggy Type, FX-1 Type, and Porsche Type, and each was available in blue, white and yellow variations, for a total of 12 different toys.
- The pre-Transformer versions of the Mini-spies were part of a toyline called Mecha Warriors, and came in blue and red variations. 
- The FX-1 type is commonly called Mazda Type by fans (as seen in the picture above), in spite of being based on an old Toyota concept car, right down to the FX-1 marking on the front of the car. This is due in equal parts to the fact that the FX-1 is an incredibly obscure car, and that the FX-1 marking is so small, it's very easy to misinterpret it as saying "RX-7" instead.