Someone logged in to my develop tenant

I just logged into my dev server and noticed that I have an error.

And after checking my tenant, I noticed that someone logged in to my dev tenant.

My login was working fine after the issue resolution. Athorization stopped saving and started asking consent on localhost - #3 by nik.baleca

But now it is not working for the local environment again.

I cleared my site data, and the error has gone. But why does it start appearing if I had not encountered it before?

And how is it possible that someone logged in via my dev environment? Have my credentials leaked?

I just created new credentials for the dev tenant. But I would like to know if it was someone from support, or if I might have leaked credentials.