Water tank false trigger
Certainly the issue is from EMI but not from the collector side as from the transistor base even the wires are shorter on that side as I understand from the comments. 

It's a common mistake to let the transistor base floating, see here a different application but with the same cause and result, the transistor is turning on apparently with no reason. With the base floating the EMI current has no other path than through the transistor base where is amplified by beta ( 110 to 800 for BC547). 

Put a 10Kohm resistor between the transistor base and emitter to  let a path for EMI currents to the ground. Try different values, smaller if you still have false triggering, higher if the sensor is not turning on the transistor. 

Of course, use  INPUT_PULLUP but the internal resistor is to high 30 to 100 kohm, better use an external resistor arround 10k or lower form the transistor collector to 3V3 to ensure a good noise margin on the collector side to. 







simulate this circuit &ndash; Schematic created using CircuitLab