https://www.amazon.com/dp/B076HMSQ2L/?tag=097-20&ascsubtag=v7_1_3_g_49nu_0_x01_-srt5-
Cheap, small, has motion detection so doesn't need to film continuously. Just leave it hooked to a power source.
Like this?
Mini Spy Hidden Camera,NIYPS 1080P Portable Small HD Nanny Cam with Night Vision and Motion Detective,Perfect Indoor Covert Security Camera for Home and Office https://www.amazon.com/dp/B076HMSQ2L/ref=cm_sw_r_cp_apa_iUCrAbZJZNA1W