What do you mean by that? Did you use any equipment to notice this?
remove signal connection to your device. If a voltmeter show 110 (or whatever the supply is) between 'hot' and the signal return coming out of the flowmeter, the signal is related to ground at the machine.

"Ground" is never "ground" there is usually (I am tempted to say always) plenty of noise between 2 places of ground.

If one side of the flowmeter signal is connected to ground, you may be able to break that connection and refer the flowmeter return to your 'box', if not ground your 'box' at the flowmeter instead of where it is located.

I hope the shield around your signal wire is only connected to ground in one end, it is a common mistake to connect it in both ends.

Erik