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The Galactic supergiant B[e] star CI Camelopardalis (CI Cam) was the firstsgB[e] star detected during an X-ray outburst. The star brightened to $\sim$2Crab in the X-ray regime within hours before decaying to a quiescent level inless than 2 weeks, clearly indicative of binarity. Since the outburst of CICam, a number of sgB[e] stars have been identified as X-ray overluminous for asingle star (i.e. $L_X > 10^{-7}~L_{bol}$). This small population has recentlyexpanded to include two Ultra Luminous X-ray sources (ULX), Holmberg II X-1 andNGC300 ULX-1/supernova imposter SN2010da. We revisit CI Cam to investigate itsbehaviour over several timescales and shed further light on the nature of thecompact object in the system, its X-ray outburst in 1998 and the binary systemparameters. We analyse archival XMM-Newton EPIC-pn spectra and light curvesalong with new data from Swift and NuSTAR. We also present high-resolutionMERCATOR/HERMES optical spectra, including a spectrum taken 1.02 days after ourNuSTAR observation. Despite being in quiescence, CI Cam is highly X-rayvariable on timescales of days, both in terms of total integrated flux andspectral shape. We interpret these variations by invoking the presence of anaccreting compact companion immersed in a dense, highly structured, asphericalcircumstellar envelope. The differences in the accretion flux and circumstellarextinction represent either changes in this environment, triggered by variablemass loss from the star, or the local conditions to the accretor due to itsorbital motion. We find no evidence for pulsations in the X-ray light curve. CICam has many similarities with SN2010da across mid-IR, optical and X-raywavelengths suggesting that, subject to distance determination for CI Cam, ifCI Cam was located in an external galaxy its 1998 outburst would have led to aclassification as a supernova imposter.

CI Camelopardalis: The first sgB[e]-High Mass X-ray Binary Twenty Years on, a Supernova Imposter in our own Galaxy?