Etheric beam locator

An etheric beam locator was a device frequently carried by the Doctor. It was presumably used to locate etheric beams, and was also useful for detecting ion-charged emissions.

When captured by Kaleds on Skaro, the Fourth Doctor had an etheric beam locator in his pocket. (DW: Genesis of the Daleks)

When faced with a bomb, the Sixth Doctor searched his pockets for an etheric beam locator. (BFA: The Condemned)

The Doctor later searched for a type 7 enhancer for his etheric beam locator. (BFA: The Raincloud Man)

While in Hexen Bridge, the Seventh Doctor had a sub-etheric beam locator in his pocket. (PDA: The Hollow Men)

On Hyspero, Sam and Gila traded a dismantled a useless etheric beam locator for some bread. (EDA: The Scarlet Empress)

While on Janus Prime, the Eighth Doctor had a sub-etheric beam locator in his pocket. (EDA: The Janus Conjunction)

Behind the scenes
The non-canonical Curse of Fatal Death had its version of the Master outfitted with etheric beam locators, which were "Dalek bumps" placed in such a way that they appeared to give the Master "breasts".