I added this bike alarm several weeks ago for some added confidence when I lock my ebike up outside local stores that have no bike racks or other solid objects to lock up to.
Amazon.com: Wsdcam 113dB Bike Alarm Wireless Vibration Motion Sensor Waterproof Motorcycle Alarm with Remote : Automotive
www.amazon.com
It fits under the saddle between the rails. I secured it to the saddle with velcro and a zip tie between the rails over the alarm. Can't see it unless you look under the saddle. It's controlled by a small remote.
I forgot about the alarm when I came up to my bike early on. Just laying my water bottle on the rack set it off in its 'watch it buddy' mode; a couple of quick loud beeps to let you know it's there.
If you move the bike again the alarm goes off until someone with a remote shuts it off.
I like it. For under $20, it's worth a look.