SMS text field filled in from unknown source

[Canopus]

I thought I had a sms text from someone I was texting to, then I realised the text field had been filled in from an unknown source, ready to send. I had not filled it in, it wasn't on my clipboard, I had not been reading anything like the text so couldn't accidentally quietly send something like what I saw. How could this happen? 

[BandOfBrothers]

This does sound like some form of glitch @Canopus 

Use > Clear Cache Files from the phone's System Partition Section  > https://www.samsung.com/uk/support/mobile-devices/how-do-i-clear-the-cache-on-my-device/ You may need to insert your earphones or hook upto a powered On Laptop or use a Tv's USB Port or a usb-c to usb-c phone to phone set up to access the Clear Partition Section. This may help with lag or stutters as this clears the phone of old/unused or broken cache files.There is also a Repair Apps option in the Menu System too { Possibly excluding the s²⁴ Series of phone's } 

If this happens again you could run a virus checker.

[Reylob]

Hi @Canopus 

Have you installed any recent app?

Which Messages app are you using?

If using Messages from Google or Samsung delete the cache from the app.

Settings -> Apps. -> Messages  and tap Force stop (right bottom corner), then

Tap Storage and tap Clear cache (right bottom corner).

If you are using a different messaging app, then clear the cache for it, the procedure is the same.

I agree with @BandOfBrothers that sounds like a glitch or a virus.

[Canopus]

I think it may very well be a glitch. It appeared to happen seconds after turning WiFi on which is set for WiFi calling. I'm going to see if it happens again and if it does I will delete caches and even clear the cache partition.