The NWS issued a flash flood watch, upgraded minutes later to a flash flood warning, just after 1am. At 4am, the NWS Austin office issues an emergency bulletin saying "This is a FLASH FLOOD EMERGENCY for South-central Kerr County, including Hunt. This is a PARTICULARLY DANGEROUS SITUATION. SEEK HIGHER GROUND NOW!"[0] These warnings would have triggered an alert on people's cell phones, however the timing makes it unlikely that campers or counselors would have been awake to notice. If someone was tasked with monitoring the weather, they either didn't get that warning or didn't act in time for it to make a difference.
There were not sirens installed to warn campers of flooding. There was a warning system installed in the late 80s, but it was shut down in 1999 after falling into disrepair due to a lack of maintenance. Efforts to install a new system with warning sirens failed to gain traction.[1]
[0]
https://www.statesman.com/story/news/nation/2025/07/08/texas...
[1]
https://www.cnn.com/2025/07/11/us/camp-mystic-owner-warnings...