  Long-lived massive magnetars are expected to be remnants of some binary
neutron star (BNS) mergers. In this paper, we argue that the magnetic powered
flaring activities of these merged magnetars would occur dominantly in their
early millisecond-period-spin phase, which is in the timescale of days. Such
flares endure significant absorption by the ejecta from the BNS collision, and
their detectable energy range is from 0.1-10 MeV, in a time-lag of $\sim$ days
after the merger events indicated by the gravitational wave chirps. We estimate
the rate of such flares in different energy ranges, and find that there could
have been ~0.1-10 cases detected by Fermi/GBM. A careful search for $\sim10$
milliseconds spin period modulation in weak short gamma-ray bursts (GRBs) may
identify them from the archival data. The next generation MeV detectors could
detect them at a mildly higher rate. The recent report on the
Quasi-Period-Oscillation found in two BASTE GRBs should not be considered as
cases of such flares, for they were detected in a lower energy range and with a
much shorter period spin modulation.
