Every ContentTranslation unit test has this notice in the log, although it does not cause the test to fail. See https://integration.wikimedia.org/ci/job/mwext-qunit/8882/consoleText for example.
Notice: Cannot find site jenkins_u3_mw in sites table [Called from Wikibase\Client\WikibaseClient::newSiteGroup in /mnt/jenkins-workspace/workspace/mwext-qunit/src/extensions/Wikidata/extensions/Wikibase/client/includes/WikibaseClient.php at line 531] in /mnt/jenkins-workspace/workspace/mwext-qunit/src/includes/debug/MWDebug.php on line 300
Call Stack
# Time Memory Function Location
1 0.0020 231032 {main}( ) ../index.php:0
2 0.2433 4761080 MediaWiki->run( ) ../index.php:43
3 0.2434 4762224 MediaWiki->main( ) ../MediaWiki.php:474
4 0.6363 18845832 OutputPage->output( ) ../MediaWiki.php:707
5 0.6502 19584184 SkinTemplate->outputPage( ) ../OutputPage.php:2321
6 0.6502 19587720 SkinTemplate->prepareQuickTemplate( ) ../SkinTemplate.php:240
7 0.9600 26673120 Hooks::run( ) ../SkinTemplate.php:472
8 0.9601 26674048 call_user_func_array ( ) ../Hooks.php:195
9 0.9601 26674912 Wikibase\ClientHooks::onSkinTemplateOutputPageBeforeExec( ) ../Hooks.php:195
10 0.9703 26712616 Wikibase\Client\WikibaseClient->getLangLinkSiteGroup( ) ../WikibaseClient.hooks.php:301
11 0.9703 26712704 Wikibase\Client\WikibaseClient->getSiteGroup( ) ../WikibaseClient.php:510
12 0.9703 26712848 Wikibase\Client\WikibaseClient->newSiteGroup( ) ../WikibaseClient.php:549
13 1.0046 26890928 wfWarn( ) ../WikibaseClient.php:531
14 1.0046 26891208 MWDebug::warning( ) ../GlobalFunctions.php:1216
15 1.0057 26893016 MWDebug::sendMessage( ) ../MWDebug.php:155
16 1.0057 26893376 trigger_error ( ) ../MWDebug.php:300
This was spotted in T120349#1851758 but was not acted on.
T102854 claims a same or similar issue has been fixed.