"I hope there are USP owners here with a similar indication in their manuals... "
I just downloaded a manual for my USP Compact in 9mm and it states,
"The HK USP Compact is a small frame pistol capable of firing the most powerful cartridges in .45ACP, .40 S&W, .357 SIG, and 9mm x 19; including all +P 9mm ammunition."
No mention of +P+ that I have found.