This app was mentioned in 1 comment, with an average of 3.00 upvotes
Nothing new https://play.google.com/store/apps/details?id=com.boxen.rs.rustysheriff&hl=en
Here.. that App is more than a year old and does EXACTLY that.. alerting you when your claim is crossed ingame
EDIT: Sorry guys, wrong APP.. THIS https://play.google.com/store/apps/details?id=com.jvetech.rustysheriffanti_raid&hl=en is the right one. Same name >.>. The other one was the Server managing App from the same guys I guess.
EDIT2: Since I haven't played Rust in nearly a year, the app was used when I played it. But it seems it's unmaintained nowadays. Sad tho. BTW: Here is the Oxide mod for it, if anyone is interested: http://oxidemod.org/threads/rusty-sheriff-raid-alert-unmaintained.8420/