Why does my girlfriend take her phone everywhere with her in the flat?

Whenever my girlfriend goes anywhere in our flat, she always takes her phone with her... everywhere (even to the toilet.)

I trust her and know she isn't unfaithful but this really upsets me and makes me wonder just why she has to take her phone everywhere with her?

I have asked her why but she just says its because she doesn't want to lose her phone.
Why does my girlfriend take her phone everywhere with her in the flat?
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