For stealth planes that can be detected but when it is too late to react. Mass production of new underwater and aerial drones with new sensors and assistance from persistent monitoring from thousands to tens of thousands of small satellites means submarines and stealth fighters will be detectable.
Teledyne Webb have designed a torpedo shaped drone submarine with a thermal engine which can extract energy from the temperature difference at different depths and continue indefinitely going at about 2 knots. Their current drone submarine can operate for months.
A thesis on gliders by William Parker for the Naval Postgraduate School simply puts it:
"A fleet of gliders could also provide a low cost network for determining the patrol patterns of threat submarine fleets."