Can't find non-invasive dc milliamp sensor
Have you tried a reed switch? They generally switch with 20 amp-turns, so you would have to wind the wire you are trying to sense current in 20÷0.3 = 67 times around the reed switch. Is your circuit OK with the added inductance?
Alternately, add &quot;process loop&quot; to your search term. Process loops run at 4-20mA, and there are current clamps to measure that non-invasively. Often, they go higher to 100mA. In your case, the 300mA won't be accurately measured, but you only want to detect the current, not measure. I imagine there must be always-on clamp sensors for current loops.