The Internet of Things (IoT) has been rapidly taking steps towards commercialization. However, the dense deployment of IoT nodes - that may follow different wireless technologies - in the shared spectrum creates a new challenge to solve: secure coordination among co-located IoT nodes from different IoT networks. In this paper, we shed light on this unique challenge, and we illustrate how this challenge has the potential to create a novel vulnerability where an attacker can pose as a hidden terminal (by manipulating its radiation patterns) and interfere with transmissions from its hidden counterparts, namely hidden terminal emulation (HTE) attack. As the dense deployment of IoT nodes will aggravate such hidden terminal interference, it facilitates the HTE attacker plausible deniability to interfere with its hidden counterparts. This paper is the first to present a theoretical analysis of the feasibility of HTE attacks (i.e., successful impersonation of hidden terminals), to illustrate how it is affected by the density of IoT nodes, and to provide insights on secure IoT deployment.