I believe it was someone working for the Trump campaign tasked with doing a review of security.
Dan Bongino mentioned the name last night. But I’m afraid I’ve forgotten the name of who put Donald wise.
The reporters who were wiretapped and surveilled by Obama also said they knew. Their computers would turn on by themselves late at night, and they heard interference on their phone.
Apparently Obama's 'spy team' are not very good at it. Someone could have told Trump, but my guess is he already knew. Any billionaire, all of them, are going to have top-notch security, including cyber security and the ability to find surveillance devices.
Kellyanne said something about this. She got it wrong with saying the microwave was bugged, but what she was trying to say I think, is that they detected microwave communications transmissions coming from their offices. Which would indicate surveillance devices are present.
I'm sure Trump had his office in Mar-a-Lago disassembled to make sure Obama's spy team had not gotten there before him. And he was likely using a secure phone service which even the NSA cannot tap.