Reported here https://github.com/bitcoin/bitcoin/pull/12629#issuecomment-381095603 by @Empact.  I've seen it now in a few different travis jobs, including:

https://travis-ci.org/bitcoin/bitcoin/jobs/366157920
https://travis-ci.org/bitcoin/bitcoin/jobs/366139851

From my initial investigation it seems like there may be an actual bug in block announcement, so I'm  opening this issue to track.
mempool.AddTransactionsUpdated(1);
    {
        WaitableLock lock(g_best_block_mutex);
        g_best_block = pindexNew->GetBlockHash();
        g_best_block_cv.notify_all();
    }
    std::vector<std::string> warningMessages;
    if (!IsInitialBlockDownload())
    {
        int nUpgraded = 0;
        const CBlockIndex* pindex = pindexNew;
        for (int bit = 0; bit < VERSIONBITS_NUM_BITS; bit++) {
            WarningBitsConditionChecker checker(bit);
            ThresholdState state = checker.GetStateFor(pindex, chainParams.GetConsensus(), warningcache[bit]);
            if (state == ThresholdState::ACTIVE || state == ThresholdState::LOCKED_IN) {
                const std::string strWarning = strprintf(_("Warning: unknown new rules activated (versionbit %i)"), bit);
                if (state == ThresholdState::ACTIVE) {
                    DoWarning(strWarning);
                } else {
                    warningMessages.push_back(strWarning);
                }