Integrated sensing and communication (ISAC) has emerged as a promising technology that enables the simultaneous operation of sensing and communication functions. By sharing hardware infrastructure, spectrum resources, and signal waveforms, ISAC can reduce hardware costs and enhance spectral efficiency. However, existing studies have ignored the interference caused by echo signals from untargeted sensing objects (SOs) in large-scale ISAC networks. Therefore, this paper investigates the ISAC coverage probability, defined as the weighted average of the probabilities that communication and sensing signal-tointerference ratios exceed their corresponding thresholds, in large-scale ISAC networks with multiple SOs. Firstly, the sensing channel gains are approximated by Gamma random variables using a moment matching method. Secondly, the analytical expressions of communication and sensing coverage probabilities are derived using stochastic geometry and validated by Monte Carlo simulations. Equipped with these results, we numerically analyze the effects of the number of antennas at base stations (BSs), the BS density, and the numbers of communication users (CUs) and SOs on the ISAC coverage probability. The numerical results show that the echo-signal interference from untargeted SOs significantly impacts the ISAC coverage probability, which decreases the ISAC coverage probability by up to 19% compared to scenarios without this interference. This indicates that the echo-signal interference cannot be ignored in ISAC networks, and its negative impact can be significantly reduced by increasing receiving antenna numbers at BSs.