Abbreviation of sentences in short notes
If you are writing or sending a short message, you often don't have long or you want to respond quickly to someone else's note. Therefore, you keep the text relatively short. A typical feature of such texts is that certain words can be omitted. The subject Ich is often left out, and then the verb is given a capital letter.
(Ich) Bin gerade im Supermarkt.
(Ich) Kann heute nicht kommen.
(Ich) Warte auf dich.
This only happens in informal short messages and notes. In all other texts, the subject cannot usually be left out.