Two things we use on the bench often enough that it was worth building them properly.
Both run entirely in your browser. Nothing is uploaded, nothing is kept, and there is no account.
Runtime and storage calculator
Every covert recorder has two clocks running against it, the battery and the card, and only one of them matters: whichever runs out first. Manufacturers quote the flattering one. Put in the cell size, the draw and the bitrate and this tells you which clock you are actually on, and by how much.
Room sweep checklist
Nineteen steps in the order they should be done, from confirming you are allowed to search the room through to writing down what you checked. Optics before radio, because a torch finds the cheap devices and an RF detector does not.
A note on what this site is for
Everything here is written for people checking their own space, their own vehicle, or a rental they are staying in. Recording other people without their knowledge is illegal in most places, and the rules on audio are stricter than the rules on video almost everywhere. If you are unsure whether something you are planning is lawful where you live, that is a question for a lawyer and not for a review site.