There's a "Global Infrasound Network" that monitors for nuclear explosions. If a reentry is big enough, I expect they'll "hear" it. It might have to be pretty big, perhaps larger than the the vast majority of meteors and satellites, but they did detect the 2013 Russian meteor.
NASA provides a summary of "fireballs" seen by "various government sensors", most likely IR-viewing satellites. (The map is not as up to date as the table on that page) It'll be interesting to see if/when Tiangong 1 shows up in the table.
Here's the announcement of tracking Tiangong 1's reentry using "Space Surveillance Network sensors and their orbital analysis system":
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TEXT archived here:
JFSCC tracks Tiangong-1’s reentry over the Pacific Ocean
By Major Cody Chiles, JFSCC Public Affairs / Published April 01, 2018
VANDENBERG AIR FORCE BASE, Calif. -- U.S. Strategic Command’s (USSTRATCOM) Joint Force Space Component Command (JFSCC), through the Joint Space Operations Center (JSpOC), confirmed Tiangong-1 reentered the Earth’s atmosphere over the southern Pacific Ocean at approximately 5:16 p.m. (PST) April 1, 2018.
The JFSCC used the Space Surveillance Network sensors and their orbital analysis system to confirm Tiangong-1’s reentry, and to refine its prediction and ultimately provide more fidelity as the reentry time approached. This information is publicly-available on USSTRATCOM’s website www.Space-Track.org.
The JFSCC also confirmed reentry through coordination with counterparts in Australia, Canada, France, Germany, Italy, Japan, South Korea, and the United Kingdom.
“The JFSCC works alongside government, industry and international partners to track and report reentries, to include today’s Tiangong-1 reentry, because the space domain is vital to our shared international security interests,” said Maj. Gen. Stephen Whiting, Deputy Commander, JFSCC, and Commander, 14th Air Force. “One of our missions, which we remain focused on, is to monitor space and the tens of thousands of pieces of debris that congest it, while at the same time working with allies and partners to enhance spaceflight safety and increase transparency in the space domain.”
The JFSCC, in conjunction with the 18th Space Control Squadron, is committed to promoting a safe, stable, sustainable, and secure space environment through space situational awareness information sharing. The command shares space situational awareness information and services with government and non-U.S. government entities that need to transit through and operate safely within the space domain. The JFSCC also shares SSA information with partners and space-faring entities to promote the responsible, peaceful, and safe use of space and to strengthen cooperation within the space domain.
“All nations benefit from a safe, stable, sustainable, and secure space domain,” said Whiting. “We’re sharing information with space-faring nations to preserve the space domain for the future of mankind.”
The JFSCC does not predict or track what happens after decay and reentry occurs, such as the exact location of fallen debris on the earth’s surface; however, the JFSCC does provide operational information about reentries and potential threats to geographic combatant commands for national defense planning and operations. Additional information about JFSCC processing and reporting reentries of space objects is available here.
For additional information about China’s Tiangong-1, please contact the China National Space Administration (CNSA).