What does I1001 mean in a METAR remarks?
It indicates ice accretion (buildup). This seems to be very obscure, I finally found it in some NOAA release notes for ASOS software v3.07:


  
  (S01126) ADD ICE ACCRETION REMARK TO METAR/SPECI REPORTS
  
  
  Discussion: The amount of ice accreting on a flat surface (planar
  icing) as estimated by the data from the freezing rain sensor, will be
  included in the remarks section of the METAR / SPECI reports. The
  remark will have the format of “Ihnnn,” where “h” is the hourly time
  period, i.e., 1, 3 or 6 hours. The value “nnn” is the ice thickness
  accumulated to the nearest 0.01 inch. 
  
  Please Note: In situations when FZRASN is manually entered in the
  Present Weather field, an inappropriate trace accretion amount (I1000,
  I3000, I6000) might be entered in the remarks section of the METAR /
  SPECI reports. This is a known issue and will be addressed at a later
  time.


Based on that, "I1001" means ice buildup of 0.01 inches in the last 1 hour.

And thanks to Jimmy for finding a more official source: FCM-H1-2017, Federal Meteorological Handbook No. 1 - Surface Weather Observations and Reports (p. 12-18) from OFCM, the Office of the Federal Coordinator for Meteorology.