This is a Brennenstuhl electronic multi-tester MT 6 S/2.
It is used for the non-contact location of cables with AC voltage. It has a checking phase for neutral and earthing conduction at sockets. There is also a continuity check (used for fuses, bulbs, etc) and a polarity check used usually for batteries). The Multi-Tester shows results through an LED.
The Multi-Tester is both small and very portable (about the same size as a pen) and it has a pocket clip. Because of the super-bright LED, it can also be used as a torch. Included are 3 replaceable coin cell batteries.