It's very hard to keep your identity completely hidden. Probably impossible to have it exactly 100%.
There are cameras that could be used to facially recognize you. Your disposable phone, unless it has all antennas removed, will send and receive information from and to the service provider it's been registered to (and even without it, AFAIK, a phone with battery in it could still send and receive information - especially if it's a smartphone, and assuming you're using Bitcoin with it... it most likely is). Just think about it - even without a SIM card you can still call 911, right? Even if you turn your phone to Airplane Mode, do you have any guarantee that it has terminated any communication?
A complete anonymity is just a dream now a days especially if the authorities are targeting you. The speaker in your phone can be activated and if the authorities knows your IMEI number of your mobile they can activate your mobile speaker even if you turned your phone to airplane mode your communications can be captured.
When it comes to bitcoin it is impossible to stay fully anonymous if you are using exchanges because they force us to verify our identity, if you are making money online and if you are accepting it in bitcoin and not using the traditional exchanges it is possible to be anonymous.