  Ultra-diffuse galaxies (UDGs) are attractive candidates to probe cosmological
models and test theories of galaxy formation at low masses; however, they are
difficult to detect because of their low surface brightness. In the Local Group
a handful of UDGs have been found to date, most of which are satellites of the
Milky Way and M31, and only two are isolated galaxies. It is unclear whether so
few UDGs are expected. We address this by studying the population of UDGs
formed in hydrodynamic constrained simulations of the Local Group from the
HESTIA suite. For a Local Group with a total enclosed mass $M_{\rm LG}\!\left(<
2.5\, {\rm Mpc}\right)=8\times10^{12}\, {\rm M_\odot}$, we predict that there
are $12\pm3$ isolated UDGs (68% confidence) with stellar masses $10^6 \leq
M_\ast\, /\, {\rm M_\odot} < 10^9,$ and effective radii $R_{\rm e} \geq 1.5\,
{\rm kpc}$, within 2.5 Mpc of the Local Group, of which $2^{+2}_{-1}$ (68%
confidence) are detectable in the footprint of the Sloan Digital Sky Survey
(SDSS). Accounting for survey incompleteness, we find that almost the entire
population of UDGs in the Local Group field would be observable in a future
all-sky survey with a depth similar to the SDSS, the Dark Energy Survey, or the
Legacy Survey of Space and Time. Our results suggest that there is a population
of UDGs in the Local Group awaiting discovery.
