The Japanese Air Force announced https://www.mod.go.jp/asdf/news/houdou/H31/310409_2030.pdf
[The following has been translated from Japanese by Pete]:
On the disappearance of the radar track of Misawa base station F-35A
(First report) Today, April 9 (Tuesday), the radar track of F-35A belonging to Misawa base disappeared. We are currently searching for it.
1 Date of occurrence: 19:27 April 9 (Tuesday)
2 Occurrence place: Aomori Prefecture Eastern Pacific Ocean (approximately 135 km east of Misawa Air Base)
3 unit name: Third Air Corps 302th Squadron
4 models, number of machines, etc: F-35A x 1 machine (1 crew)
5 Situation of the pilot: Details are under review.
6 There is no information on damage information at the moment.”
The Japanese Ministry of Defense [has suspended] flights of all remaining F-35As at the Misawa Air Base for the time being, Iwaya added.
The Japanese F-35 fighter jet missing in the Pacific could be a major security headache for the U.S. if Russia or China locate the state-of-the-art fighter jet first, experts warn.
COMMENT
Japan's (and US joint) Misawa Air Base is on the northern tip of Japan's home island of Honshu (see Misawa in red on map below)
One would normally expect some emergency signal, radar and/or satellite signals if the F-35 crashed.
Has the pilot defected with his F-35?
Pete