The above only (with one exception) apply if the device can transmit. Airplane mode is about the only thing that somewhat reliably stops them from doing so - it's mandated by the US FAA, due to concerns that radio frequency transmissions could harmfully interact with aircraft nav and flight control systems. I believe, but do not know, that Canadian aviation authorities have similar rules for devices.
The exception, of course, is if the device is hacked. Any software-controlled feature (such as airplane mode) can be bypassed this way. The only secure safeguard is a physical switch that interrupts a wire. One of my laptops has that; an airplane mode physical switch breaks the connection to the WiFi antenna, so it's not vulnerable to a software hack bypass.
As for the phone call to the taxi driver; was it ever confirmed that was the suspects? Even if so, and even if the authorities were able to figure out a location via cellular records, they might only have had the phone on for a few minutes, which would tell the authorities where the phone was at that point in the past.
Basically, if something isn't transmitting, you can't track it.