I recently wrote a REST API for a personal project amongst friends. I sent a sample endpoint URL (i.e. à la https://my-api.com/api/resource/4/something) to one of my collaborators over SMS via Messages by Google on Android. Immediately upon sending the SMS, three GET requests came in to my server for that resource. After tracing the IP addresses, (66.102.6.48, 66.249.80.97, 74.125.210.60), I found out that they are from a Google Proxy in Mountain View.
Furthermore, this occurs again each time I send a URL over SMS in this manner.
I am curious why this might be happening? My current suspicion is that Google is doing some sort of caching or minimization. Anybody know?
The reason may be simpler than you think.
In most messaging platforms, when you send a link they make a GET request to the server to display a preview of the website, think about what happens when you share a link on WhatsApp.
It may very well be the application on your smartphone and/or the one of the receiver, trying to fetch a preview for that link.
Try sending it from a different kind of platform, application or phone (do you have an old Nokia sitting around?). My guess is that you won't see that GET request!